This invention relates to watermarked digital images. In particular, it relates to methods for improving confidence in and for authentication of watermarked digital images.
In order to increase confidence of use of digital images as evidence, possibly in a court of law, there is a significant need to demonstrate that an image has not been tampered with.
It is known to use audit trails, in which information about when an image was processed is appended to the image, but these methods are only applicable once an image has been registered onto a system. Such audit trails therefore cannot detect any unauthorized operations prior to registration on a computer, and may not be able to report on the type of processing done at any one time. Audit trails can also be avoided or corrupted, whether deliberately or accidentally.
Image watermarking is a known technique. In this technique, a known binary pattern or signature is embedded into an image at the moment of image acquisition. Such watermarks are called xe2x80x9crobustxe2x80x9d because they are designed to remain intact regardless of any post-processing of the image such as filtering, cropping etc. While such watermarks do provide a useful degree of protection, they can at present not be wholly relied on and they cannot always possess the required degree of surety that an image has not been tampered with in order for the image to be used as evidence under the strict rules of courts of law, etc.
Ruanaidh, Dowling and Boland xe2x80x9cPhase Watermarking of Digital Imagesxe2x80x9d, IEEE INTCONF Image Processing, Vol. 3, Lausanne, Switzerland, September 1996, pp 239 to 241, describes a technique for watermarking digital images in which an image is divided into blocks of a selected size (e.g. 16xc3x9716 pixels). A discrete Fourier transform (DFT) is applied to the luminance component of the image on a block by block basis. The DFT is a complex value and thereby generates a modulus and a phase. The resulting watermark comprises a binary string of 1""s and 0""s which may represent, for example, a company logo, a user authentication code, date/time/location information and so on. The watermark is embedded in the image by altering the phase of selected DFT coefficients.
The present invention arose in an attempt to provide an improved method of authenticating, and thereby improve confidence in, a watermarked image.